Project Summary/Abstract ? Beard Over the past decade, evidence has been mounting concerning the importance of the extracellular matrix (ECM) in development and maintenance of cellular differentiation, especially in endothelial cells that form the blood-brain barrier (BBB). Alterations within the ECM can negatively impact normal cellular functions, such as the upregulation and localization of occlusive proteins to endothelial cell-cell contacts where they form junctional complexes to prevent passage of fluids, solutes, and cells through paracellular pores. Tight junction (TJ) proteins, such as claudin-5 (CLDN5), are critical for sealing these gaps and forming the BBB. Under normal conditions, ECM proteins in the basement membrane contribute to enhancing the barrier properties of the BBB, as evidenced by a more BBB-like phenotype in brain endothelial cells when cultured on substrates comprising known BBB ECM proteins, although the mechanisms responsible are not fully understood. While several vital constituents of the ECM have been identified as necessary for BBB integrity, major gaps in knowledge include what changes within the BBB ECM occur during inflammatory injuries that result in barrier disruption, as well as how ECM-EC interactions transduce intraendothelial signaling to regulate EC TJs. Our long-term goal is to elucidate endothelial-specific signaling pathways that are responsible for BBB dysfunction during inflammation. The overall objective of this proposal is to define the role of ECM-mediated regulation of CLDN5-dependent BBB function during homeostasis and inflammation. Emphasis is placed on a novel role for the isoform-specific function of AKT2 in maintaining maximal CLDN5 expression during homeostasis, plus a proinflammatory role for two small leucine-rich proteoglycans (SLRPs), decorin and biglycan, in CLDN5 downregulation during neuroinflammation. The central hypothesis is that inflammation triggers a release of endothelial-derived SLRPs that act in an autocrine fashion to interfere with ECM-dependent regulation of CLDN5, contributing to BBB dysfunction. This hypothesis was derived from several preliminary findings generated in the applicant?s laboratory. The rationale for the proposed research is that a better understanding of matrix pathobiology will translate into a better understanding of the pathogenic role of BBB dysfunction in inflammation-associated diseases in the central nervous system (CNS), such as multiple sclerosis (MS), stroke, traumatic injuries, dementia, encephalopathies, and brain metastases, all of which collectively account for suffering of approximately 9 million people in the United States and a cost burden of over 300 billion dollars annually. The proposed research is significant, as it is expected the data derived from these studies will not only establish novel concepts in ECM-dependent regulation of endothelial cell biology, but it will also provide new mechanistic insights into the pathophysiology of BBB dysfunction with the potential to provide a basis for the development of new therapeutic targets.